If you like to disable Dark Mode for the FileMaker 19 application, but continue to use Dark Mode for all other applications, you can use the following command line in Terminal to run FileMaker always in light mode:
defaults write /Applications/FileMaker\ Pro.app/Contents/Info NSRequiresAquaSystemAppearance -boolean yes
If you have FileMaker Pro installed in a different location, you may need to adjust the path. To reverse you can always call this again with "no" as value on the end.
If a future FileMaker 19 update replaces the application, this setting is reset.
PS: This may break code signing, so you may also use codesign --remove-signature command to remove the broken signature.